Ingition

[robert dingli]

Hi guys,

I'm sorry that I was away for a couple of days earlier this week and
managed to miss the igntion driver discussion on line.  

Thanks to Ed and Frank for providing background info on the topic so
that the rest of us can contribute to the discussion from a solid
footing.  As an ignition system designer, I found the messages 
comprehensive and enlightening and at the very least, will save much
'waffle' in the future.  On the other hand, some of the replies were 
a bit painful to wade through.

Someone asked about the choice of ignition coils.  I have used many
different types from second-hand oil filled units to current model
Delco, Bosch, Ford and Nippondenso units.  Dual fire coils are my first
choice for the job (as they halve the total number of coils) but I
have had to resort to coil per plug systems at times.  Obviously, the 
lower primary impedance coils charge more quickly, but for multi coil 
system this isn't an issue as they have much more time to charge.

Something to note is that the switch-on transient when the coil starts to
charge can result in enough voltage drop across the secondary to cause a 
spark in favourable conditions.  Distributor and dual firing systems don't
have this problem due to the extra gaps in the secondary circuit.  OEM
coil-per-plug coils thus incorporate a diode in series with the secondary
to stop reverse current flow.

Lastly, someone else asked about the hazards of the wasted spark approach.
I remembered recently where I had heard of problems.  90 degree V6 engines
with only three crank throws fire unevenly 90/150/90/... degrees apart.
The wasted spark may fire while a combustable mixture is present in the
out-of-phase cylinder.   I believe the ignition timing is compromised as
a result.  Perhaps someone experienced with the Delco controlled Buick
3800 V6, as used in local Holden Commodores, may care to comment.

Enough for now.  :-)

Robert 'trying not to waffle' Dingli

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             Robert Dingli           r.dingli at ee.mu.oz.au

Power and Control Systems                 Thermodynamics Research Lab
Electrical Engineering                    Mechanical Engineering
   (+613) 344 7966                           (+613) 344 6728
  University of Melbourne, Parkville, 3052, Victoria, AUSTRALIA
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